Number-average chain length and Carothers accounting
For an ideal linear step-growth model, the number-average length Xn counts monomer-derived units per product molecule, including unreacted monomers in the counted population.
Key knowledge
For an ideal linear step-growth model, the number-average length Xn counts monomer-derived units per product molecule, including unreacted monomers in the counted population. When bifunctional A and B groups begin in equal numbers and each intermolecular A–B link reduces the molecule count by one, Xn=N0/N=1/(1−p). For an AA/BB mixture with initial group ratio r=NA,0/NB,0≤1 and p the fraction of minority A groups reacted, Xn=(1+r)/(1+r−2rp). High conversion and close stoichiometric balance are needed for large average chain length.
Values and equations
Xn=N0/N; Xn=1/(1−p) when r=1; Xn=(1+r)/(1+r−2rp)
- Xn
- number-average monomer-derived units per molecule 1
- N0,N
- initial and remaining counted molecules counts
- NA,0,NB,0
- initial minority-A and majority-B functional groups counts
- r
- initial functional-group ratio NA,0/NB,0 1
- p
- fraction of minority-A groups that have reacted 1
Conditions and scope
- Positive initial group counts, bifunctional linear monomers, only A–B linking, no cyclization, branching, chain scission, chain-stopping impurity or unaccounted molecular loss; 0<r≤1 and 0≤p≤1 with a positive denominator. The p→1,r=1 divergence is an ideal limiting result, not an attainable measured infinite molecule.
- The counted monomer-derived unit is not always one full constitutional repeat: an AA/BB repeat can contain contributions from two starting molecules. Converting Xn to molar mass must account for that convention, eliminated small molecules and end groups; Xn alone does not determine a full chain-length distribution.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read actual equal-group molecule-count derivation, minority-group ratio definition, imbalance count and final general equation plus the two-monomers-per-repeat example. Independently checked N0=(NA,0+NB,0)/2 and N=N0−pNA,0. Excluded source's approximate repeat-mass relation as an exact end-group identity and extending linear Carothers counting to a crosslinked gel. This is reference accounting rather than a synthesis recipe.
2026-10-10 · Independent AI editorial review: All 94 actual compact claims, hypotheses, equation/variable/unit tables and 176 origin semantic links independently read. All exact author source-binding rows and disclosed extents assessed; targeted original bodies or specifically identified indexed primary passages rechecked below. All retained chemistry compact records read for combined breadth. Independent AI editorial content and practical common-core inventory review; no human specialist, degree/laboratory proficiency, whole-textbook certification or experimental replication. Receipt: chemistry-independent-content-inventory-review.1.
Entry details
Permanent link to this entry ↗- Record identity
- uni-chem-step-growth-chain-accounting
- Revision
- core.1
- Structured reference record
- University core expansion